EFFECTIVE MASSES IN HIGH-MOBILITY 2D ELECTRON-GAS STRUCTURES

Effective masses derived from the low-field Shubnikov-de Haas oscillat ions in high mobility GaAs/AlGaAs samples are found to decrease system atically with density. Measurements in a low-density sample show the t emperature dependence does not conform to standard theory. It is argue d that this is to be expected when there is extra structure, in additi on to the Landau levels, in the density of states. This may be importa nt in interpreting measurements of composite fermion effective masses.